I saw this at the Daily Mail, which everyone should know is a very unreliable source, but it's still a little concerning:
They said that the super-volcano underneath the Wyoming park has been rising at a record rate since 2004 - its floor has gone up three inches per year for the last three years alone, the fastest rate since records began in 1923.
But hampered by a lack of data they have stopped short of an all-out warning and they are unable to put a date on when the next disaster might take place.
When the eruption finally happens it will dwarf the effect of Iceland's Eyjafjallajökull volcano, which erupted in April last year, causing travel chaos around the world.
The University of Utah's Bob Smith, an expert in Yellowstone's volcanism told National Geographic: "It's an extraordinary uplift, because it covers such a large area and the rates are so high."
"At the beginning we were concerned it could be leading up to an eruption."
The prior probability of a catastrophic eruption per year is about 0.00014%. Alexsei remarked in the comments that the probability is actually higher because it rises the more time elapses since the last eruption, and the eruptions have been fairly periodic for the last two million years. I want to do a more precise calculation, but say that it increased the probability by a factor of ten, that would be 0.0014% chance per year, or a 0.14% chance this century.